The occurrence of headaches as a side effect of glyceryl trinitrate (nitroglycerin) is a well-known, direct physiological consequence of its vasodilatory action and is considered common knowledge requiring no citation.
The claim states that glyceryl trinitrate patches cause severe headaches as a side effect. Vasodilation caused by nitric oxide donors frequently triggers headaches (in fact, nitroglycerin is widely used experimentally to induce migraine-like headaches in animal models, as seen in papers [2], [3], [5], [7], and [9]). This is a textbook medical fact about organic nitrates known by everyday clinical observation and pharmacology. Following the instructions for STEP 0, trivial or well-established direct observations of this nature are classified as COMMON_KNOWLEDGE, meaning no papers should be attached and no formal evidence weigh-in is necessary.